Published Version

Abstract

There have been three Cassini encounters with the south-pole eruptive plume of
Enceladus for which the Cassini Plasma Spectrometer (CAPS) had viewing in the
spacecraft ram direction. In each case, CAPS detected a cold dense population of heavy
charged particles having mass-to-charge (m/q) ratios up to the maximum detectable by
CAPS ( 104 amu/e). These particles are interpreted as singly charged nanometer-sized
water-ice grains. Although they are detected with both negative and positive net charges,
the former greatly outnumber the latter, at least in the m/q range accessible to CAPS.
On the most distant available encounter (E3, March 2008) we derive a net (negative)
charge density of up to 2600 e/cm3 for nanograins, far exceeding the ambient plasma
number density, but less than the net (positive) charge density inferred from the RPWS
Langmuir probe data during the same plume encounter. Comparison of the CAPS data
from the three available encounters is consistent with the idea that the nanograins leave the
surface vents largely uncharged, but become increasingly negatively charged by plasma
electron impact as they move farther from the satellite. These nanograins